System and method for predetermining the onset of impending oscillatory instabilities in practical devices

ABSTRACT

A system for early detection of onset of oscillatory instabilities in practical devices is described. The system consists of a measuring device, an instability detection unit and a control unit. The measuring device is configured to generate signals corresponding to the dynamics happening inside the practical device. The instability detection unit is configured to diagnose the stability of the practical device from the signals that are generated by the measuring device. Further, the control unit is configured to control various operating parameters in the practical device based on the information obtained from the instability detection unit.

PRIORITY DETAILS

The present application is based on, and claims priority from, IndianApplication Number 4110/CHE/2012, filed on 1 Oct., 2012, IndianApplication Number 4476/CHE/2012, filed on 26 Oct., 2012 and PCTApplication Number PCT/IN2013/000197, filed on 25 Mar., 2013, thedisclosure of which is hereby incorporated by reference.

FIELD OF INVENTION

The embodiments herein relate to a system and a method forpredetermining the onset of impending oscillatory instabilities inpractical devices, and more particularly but not exclusively to a systemand a method for predetermining the onset of impending oscillatoryinstabilities in devices such as high Reynolds number flow or combustiondevices and/or noisy acoustic devices, and controlling variousparameters of the device in order to prevent the device from oscillatoryinstabilities.

BACKGROUND OF INVENTION

Controlling oscillatory instabilities is very important in many devicesthat are being used in various fields because such oscillations lead toa decreased performance and reduced lifetime of such devices. In devicessuch as combustors that are used in gas turbines, jet engines, andindustrial processing devices such as furnaces and burners, controllingand avoiding the oscillatory instability remains a challenging task asthese devices are driven by a variety of flow and combustion processes.Further, in these devices, oscillatory instabilities may arise easily asonly a small fraction of the energy available to the system issufficient to drive such instabilities and the corresponding attenuationin the device is weak. Hence, large amplitude pressure oscillations areeasily established in these devices resulting in performance losses,reduced operational range and structural degradation due to increasedheat transfer. Further, detection of the onset of oscillatoryinstabilities remains a challenging task in other fields as well; forexample, flow induced vibrations due to aeroelastic instabilities andpipe tones arising due to aero acoustic instabilities.

Researchers have proposed various techniques to control oscillatoryinstabilities occurring in practical systems such as combustors andturbo machinery, some of which are listed below. In one of the proposedtechniques, a delay feedback controller is used with the combustors. Thedelay feedback controller modifies the pressure in the fuel line tocontrol instabilities. Although, the technique of using delay feedbackcontroller is partially successful in controlling instabilities incombustors, it should be noted that this technique may not be amenableto most fielded systems as it requires external actuators, modificationof combustor configuration and knowledge of frequency response for anarbitrary input. Further, the instability can be controlled only afterthe instability occurs and thus the technique fails to prevent theinstability.

In another conventional technique, the combustor stability is determinedbased on the bandwidth of the combustor casing vibration and dynamicpressure measurements in combustion chambers. The bandwidth which isindicative of the damping, decreases towards zero as the combustorsapproach the stability limits. However, the presence of noise in thecombustion chamber could make this technique partially inefficient, asit relies on frequency domain analysis.

In yet another conventional technique, the stability margin ofcombustors is determined using exhaust flow and fuel injection ratemodulation. However, this technique is again restricted by the need foracoustic drivers and pulsed fuel injectors. Another conventionaltechnique proposed a detector that utilizes autocorrelation of theacquired signal to characterize the damping of the combustor. Theinstability of the combustor is tracked by the detector when the dampinggoes to zero. This technique again requires the combustor to reachinstability for the detector to work. Further, the technique may not beeffective for combustors exhibiting pulsed instabilities and noiseinduced transition to instability. In addition, the presence of multiplefrequencies in the spectrum makes the concept of damping unclear.

In order to avoid combustion instabilities, combustor designersincorporate sufficient stability margin in the design of the combustor.The stability margins prevent instabilities from occurring even in theworst possible scenario. However, such conservative estimates onoperational regimes lead to increased levels of NO_(x) emissions makingit more difficult to meet the demanding emission norms.

In yet another conventional technique, aerodynamic and aeromechanicalinstabilities in turbofan engines are detected using a sensor positionedin the compressor portion of the engine which generates a precursorsignal to instability after passing through a carefully selectedbandpass and filter. This approach to detect instability is problematicdue to similar issues discussed in the previous systems.

Thus, the conventional techniques for controlling the oscillatoryinstabilities require either incorporation of certain design features inthe device or the incorporation of sensors or similar detectors thatcould detect the instability and control the instability. Further, boththe processes are directed to identifying the instability after theinstability occurs. Hence, there exists a need for a system and a methodthat could predetermine the instability and control various parametersof the device accordingly, to prevent the system from entering anoperational regime where it becomes unstable, thus improving thestability margins.

OBJECT OF INVENTION

The principal object of this invention is to provide a system for earlydetection of the onset of oscillatory instabilities in practicaldevices.

Another object of this invention is to provide a system for earlydetection of the onset of oscillatory instabilities in practicaldevices, and controlling various parameters of the device in order toprevent the device from developing oscillatory instabilities.

A further object of this invention is to provide methods for earlydetection of the onset of oscillatory instabilities in practicaldevices.

Yet another object of this invention is to provide methods for earlydetection of the onset of oscillatory instabilities in practicaldevices, and controlling various parameters of the device in order toprevent the device from entering an operational regime where oscillatoryinstabilities exist.

These and other objects of the embodiments herein will be betterappreciated and understood when considered in conjunction with thefollowing description and the accompanying drawings. It should beunderstood, however, that the following descriptions, while indicatingpreferred embodiments and numerous specific details thereof, are givenby way of illustration and not of limitation. Many changes andmodifications maybe made within the scope of the embodiments hereinwithout departing from the spirit thereof, and the embodiments hereininclude all such modifications.

BRIEF DESCRIPTION OF FIGURES

This invention is illustrated in the accompanying drawings, throughoutwhich like reference letters indicate corresponding parts in the variousfigures. The embodiments herein will be better understood from thefollowing description with reference to the drawings, in which:

FIG. 1 is a block diagram of a system 100 for early detection of onsetof impending instabilities in practical devices;

FIG. 2 is a graph depicting a measure based on the 0-1 test applied onthe dynamic pressure data obtained from a combustor (C) in a particularconfiguration as the parameters are moved towards instability;

FIG. 3 is a graph depicting a measure based on the number of peakscrossing a set threshold value applied on the dynamic pressure dataobtained from a combustor (C) in a particular configuration as theparameters are moved towards instability;

FIG. 4 is a graph depicting a measure based on the Hurst exponent testapplied on the dynamic pressure data obtained from a combustor (C) in aparticular configuration as the parameters are moved towardsinstability;

FIG. 5 is the schematic of the system used for the early detection ofonset of instabilities in the combustor (C), by counting the burstsgenerated within the combustor (C);

FIG. 6 depicts the schematic of the system 100 for early detection ofonset of instabilities in the combustor (C), by means of computing theHurst exponent; and

FIG. 1 is a flowchart depicting a method for early detection of onset ofoscillatory instabilities in practical devices and controlling variousparameters of the device in order to prevent the device from oscillatoryinstabilities.

FIG. 8 depicts the schematic of the system 100 for early detection ofonset of instabilities in an aeroacoustic system, by means of computingthe Hurst exponent.

DETAILED DESCRIPTION OF INVENTION

The embodiments herein and the various features and advantageous detailsthereof are explained more fully with reference to the non-limitingembodiments that are illustrated in the accompanying drawings anddetailed in the following description. Descriptions of well-knowncomponents and processing techniques are omitted so as to notunnecessarily obscure the embodiments herein. The examples used hereinare intended merely to facilitate an understanding of the ways in whichthe embodiments may be practiced and to further enable those of skill inthe art to practice the embodiments. For example, although, certainembodiments herein are related to the system and method for earlydetection of instabilities in devices such as combustors in gasturbines, and industrial processing devices such as furnaces and burnersfor the ease of understanding the invention, it should be noted that thesystem according to the present invention may also be used for any otherdevices in which the transition to oscillatory instability from chaoticbehavior happens though intermittent bursts. Further, although, certainembodiments herein are related to the system and method for detectingand preventing oscillatory instabilities in combustion systems, itshould be noted that the system and method according to the presentinvention could also be used for preventing oscillatory instability inany noisy or chaotic systems; for example, structural instabilities thatmay arise due to aeroelastic flutter or flow induced vibration,aerodynamic and aeromechanical instabilities such as surge and, orinstabilities arising in magnetohydro dynamics, or aeroacousticinstabilities in gas transport systems exhibiting pipe toneinstabilities. Furthermore, although, certain embodiments herein arerelated to the systems and methods that utilizes faster and more robusttechniques of burst counting and Hurst exponent methods for earlydetection of onset of instabilities, it should be noted that the systemcould utilize any other methods that could determine the transition toinstability through intermittent burst in a smooth manner. Accordingly,the examples should not be construed as limiting the scope of theembodiments herein.

The embodiments herein achieve a system and method for determining theoscillatory instabilities in practical devices, before the instabilityoccurs. Further, the embodiments herein achieve a system and method forpredetermining the oscillatory instabilities in practical devices andcontrolling various parameters of the device in order to prevent thedevice from oscillatory instabilities. Further, the embodiments hereinachieve a system and method for early detection of onset of oscillatoryinstabilities in devices where the transition to oscillatory instabilityfrom chaotic or noisy behavior happens though intermittent bursts, andcontrolling various parameters of the device in order to prevent thedevice from developing oscillatory instabilities. Referring now to thedrawings, and more particularly to FIGS. 1 to 6, embodiments are shownwhere similar reference characters denote corresponding featuresconsistently throughout the figures.

FIG. 1 is a block diagram of a system 100 for early detection ofinstabilities in a practical device. In an embodiment, the system 100 isconfigured to detect instabilities in devices such as combustors (C) ingas turbines, and industrial processing devices such as furnaces andburners. However, it is also within the scope of invention, that thesystem 100 could be used for any other device that encounters unwantedoscillatory instabilities without otherwise deterring the intendedfunction of the system 100 as can be deduced from this description. Thesystem 100 includes a measuring device 102, an instability detectionunit 104 and a control unit 106. The measuring device 102 is configuredto acquire signals corresponding to the dynamics happening inside thecombustor (C). In an embodiment, the measuring device 102 is configuredto acquire acoustic signals corresponding to the dynamics happeninginside the combustor (C). In an embodiment, the measuring device 102 isprovided in communication with the combustor (C) or any other devicethat has to be prevented from oscillatory instabilities. The instabilitydetection unit 104 is configured to diagnose the stability of thecombustor (C) from the signals (Φ(j)) that are generated by themeasuring device 102. The control unit 106 is configured to controlvarious operating parameters in the combustor (C) based on theinformation obtained from the instability detection unit 104.

In another embodiment, the system 100 also includes a signal conditioner108, an analog to digital convertor 128 and a digital to analogconvertor 120. The signal conditioner 108 is configured to manipulatethe signal (Φ(j)) generated by the measuring device 102, such that itmeets the requirements of analog to digital convertor 128. In anembodiment, the signal conditioner 108 is configured to amplify thesignal (Φ(j)) generated by measuring device 102. Further, if the signal(Φ(j)) obtained from the measuring device 102 is analog, the analog todigital convertor 128 coverts the analog signal to digital signal suchthat the signals (Φ(j)) could be processed in the instability detectionunit 104. Further, the digital to analog convertor 120 converts thedigital signal obtained as the output from instability detection unit104 to an analog signal such that it could be processed by the controlunit 106.

It should be noted that the aforementioned configuration of system 100is provided for the ease of understanding of the embodiments of theinvention. However, certain embodiments may have a differentconfiguration of the components of the system 100 and certain otherembodiments may exclude certain components of the system 100. Therefore,such embodiments and any modification by addition or exclusion ofcertain components of system 100 and without otherwise deterring theintended function of the system 100 as is apparent from this descriptionand drawings are also within the scope of this invention.

In an embodiment, the instability detection unit 104 diagnoses whetherthe dynamics of the combustor is chaotic/noisy or non-chaotic/periodic,based on the signals Φ(j) generated by the measuring device 102 as atime series. A mathematical method described as 0-1 test in theliterature can be used to identify the presence of chaos in a given timeseries. The instability detection unit 104 encapsulates a fundamentallynew and heretofore unexplored application of the test as a tracker ofoscillatory instabilities. The signal Φ(j) is measured such that themeasured value at each instant provides essentially no information aboutfuture values when the combustor is in a stable operating condition.This is accomplished by configuring the instability detection unit 104to sample the measured signal at a time interval corresponding to thefirst minimum of the average mutual information of the signal Φ(j). Theaverage mutual information could be obtained as

${I(\tau)} = {\sum\limits_{j = 1}^{N}\; {{P( {{\Phi (j)},{\Phi ( {j + \tau} )}} )}{\log_{2}\lbrack \frac{P( {{\Phi (j)},{\Phi ( {j + \tau} )}} )}{{P( {\Phi (j)} )}{P( {\Phi ( {j + \tau} )} )}} \rbrack}}}$

where,

I represents the average mutual information.

τ represents the location of average mutual information.

Φ(j) represents the measured signal from combustor for j=(1, 2, . . . ,N)

P(S) represents probability of the event S.

Typically, the location of the first minimum of the average mutualinformation (τ_(min)) is T/4, where T is the natural acoustic period ofoscillations in the combustor. In an embodiment, although, the value ofτ_(min) is prescribed, the instability detection unit 104 is robust forvarious values of the sampling interval as long as the consecutivevalues are poorly correlated. For example, comparable values of τ_(min)may also be obtained by using a sampling interval corresponding to thefirst zero crossing of the autocorrelation of Φ(j).

Further, from the measured signal Φ(j) for j=(1, 2, . . . , N) andj_(i+1)−j_(i)=τ_(min), translation variables p_(c) and q_(c) is obtainedas,

${p_{c}(n)} = {\sum\limits_{j = 1}^{n}\; {{\Phi (j)}{\cos ({jc})}}}$${q_{c}(n)} = {\sum\limits_{j = 1}^{n}\; {{\Phi (j)}{\sin ({jc})}}}$

where c is chosen randomly in the interval (π/5, 4π/5). The diffusive(or non-diffusive) behavior of p_(c) and q_(c) can be investigated byanalyzing the mean square displacement M_(c)(n). If the dynamics isregular then the mean square displacement is a bounded function in time,whereas if the dynamics is chaotic then the mean square displacementscales linearly with time. The mean square displacement M_(c) (n) of thetranslation variables could be computed as

${M_{c}(n)} = {{\lim\limits_{Narrow\infty}{\frac{1}{N}{\sum\limits_{j = 1}^{n}\; \lbrack {{p_{c}( {j + n} )} - {p_{c}(j)}} \rbrack^{2}}}} + \lbrack {{q_{c}( {j + n} )} - {q_{c}(j)}} \rbrack^{2}}$

Note that this definition requires n<<N, where N represents the size ofthe measured signal. Further, the limit is assured by calculatingM_(c)(n) only for n≦n_(cut) where n_(cut)<<N. In practice, we find thatn_(cut)=N/10 yields good results, where n_(cut) represents the value ofthe index up to which mean square displacement M_(c)(n) is calculated.The test for chaos is based on the growth rate of M_(c) (n) as afunction of n. Hence, in order to formulate a modified mean squaredisplacement D_(c)(n) which exhibits the same asymptotic growth as M_(c)(n) but with better convergence properties, the instability detectionunit 104 is configured to remove the oscillatory term V_(osc)(c, n) fromthe mean square displacement M_(c)(n). The modified mean squaredisplacement D_(c) (n) could be obtained as

D_(c)(n) = M_(c)(n) − V_(osc)(n) where${V_{osc}( {c,n} )} = {( {E\; \Phi} )^{2}\frac{1 - {\cos ({nc})}}{1 - {\cos (c)}}}$and${E\; \Phi} = {\lim\limits_{Narrow\infty}{\frac{1}{N}{\sum\limits_{j = 1}^{n}\; {\Phi (j)}}}}$

Hence, by defining vector ξ=(1, 2, . . . , n_(cut)) and Δ=(D_(c)(1),D_(c)(2), . . . , D_(c)(n_(cut))), the asymptotic growth rate K_(c) ofthe modified mean square displacement D_(c) with n could be obtainedfrom the correlation of the vectors and ξ and Δ. Normally, the value ofK_(c) essentially allows the user of the system 100 to distinguishbetween the chaotic and non-chaotic dynamics of the combustor. Theasymptotic growth rate K_(c) is a function of c for regular and chaoticdynamics. In the case of periodic dynamics, most values of c yieldK_(c)=0 as expected, but there are isolated values of c for which K_(c)is large. Therefore, to ensure robustness of the measure to outliers andspurious resonances, the median value of K_(c) (say K) is obtained fordifferent random values of c. The obtained value of K would lie close to1 for noisy/chaotic signals and close to 0 for regular dynamics.Further, if the combustor flow field is inherently turbulent, thetransition to instability would be associated with a decrease in thevalue of K from 1 to a lower value depending on the turbulent intensity;i.e., higher the intensity of turbulence at instability, higher thedeparture of K from 0 at instability. Hence, a threshold value of K maybe defined upon crossing of which a suitable control unit 106 may beconfigured to control various parameters of the combustor and maintainthe combustor under stable operating conditions. FIG. 2 is a graphshowing the results of the instability detection unit based on this testapplied on the dynamic pressure data obtained from a combustor in aparticular configuration as the parameters are moved towards oscillatoryinstability.

In another embodiment, the system 200 is provided with a unit for theearly detection of onset of instabilities by explicitly tracking theintermittent bursting behavior preceding the transition to instabilityfrom chaos. The system 200 includes a measuring device 202, aninstability detection unit 204 and a control unit 218. The measuringdevice 202 is configured to generate signals (Φ(j)) corresponding to thedynamics happening inside the combustor (C). In an embodiment, themeasuring device 202 is configured to generate acoustic signalscorresponding to the dynamics happening inside the combustor (C). In anembodiment, the measuring device 202 is provided in communication withthe combustor (C) or any other device that has to be prevented fromoscillatory instabilities. The instability detection unit 204 isconfigured to diagnose the stability of the combustor (C) from thesignals (Φ(j)) that are generated by the measuring device 202. In anembodiment, the instability detection unit 204 is a programmed unit thatrequires the sampling rate (F_(s)) for which the signal Φ(j) isobtained. In an embodiment, the signal Φ(j) could be acquired by fixing,=10F_(max), (where F_(max) is the maximum frequency one wishes toprevent), as any device generally starts the operation at a stablecondition. The sampling rate (F_(s)) at which the signal Φ(j) can beacquired is related to the location of the first minimum of averagemutual information (τ_(min)). After computing τ_(min), the sampling rate(F_(s)) at which the signal Φ(j) is acquired could be then revised asF_(s)=10/τ_(min). By fixing the sampling rate (F_(s)) the system couldbe optimized for precursor detection. It should be noted that theaforementioned procedure for obtaining τ_(min) and F_(s) is provided forthe ease of understanding of an embodiment of the invention. Further,although the aforementioned values of τ_(min) and F_(s) are prescribed,it should be noted that the detection techniques utilized in the system100 as disclosed in this description are robust for changes in thesequantities within a reasonable range.

The system 200 includes at least one sensor. The sensor is configured toacquire signal from the device (combustor (C)) to which the system 200is incorporated. The acquired signal then reaches the instabilitydetection unit 204 where the proximity of the operating condition toinstability is determined. Further, the instability detection unit 204is configured to generate appropriate signals corresponding to theinstability and transfers the signals to the control unit 218.

The control unit 218 is configured to control various operatingparameters in the combustor (C) based on the information obtained fromthe instability detection unit 204. In an embodiment, a suitablethreshold is set for the number obtained by the instability detectionunit 204, such that when the threshold is crossed, the control unit 218suitably ensures that the combustor (C) remains in stable operatingconditions, by controlling various parameters in the combustor (C),thereby increasing the stability margin of the combustor (C).

The instability detection unit 204 diagnoses the early detection of theonset of instabilities in the device (combustor (C)) to which the system200 is incorporated, by examining the bursts generated within the deviceprior to instability. Bursts refer to a sudden spike in the amplitude ofthe measured signal which decays after a short duration. The occurrenceof such bursts in the measured signal leads to an intermittent switchingbehavior of the signal between low and high amplitudes. This is oftenthe case in high Reynolds number flow devices where the transition tooscillatory instability from chaotic behavior happens throughintermittent bursts. Such bursts are also common in systems with highlevels of noise where the transition to instability happens through aregion characterized by intermittent bursts.

In one embodiment, the onset of impending instabilities is determined bycounting the number of peaks (N) in the signal Φ(j) above a user-definedthreshold (ξ) for a time duration (t). The threshold (ξ) wouldcorrespond to the acceptable levels of amplitude of the device(combustor (C)). In an embodiment, the value of time duration (t) isdefined as 400τ_(min) and all the peaks (N_(tot)) that are generatedwithin the time duration (t) are counted. In an embodiment, the timeduration (t) would correspond to 100 oscillatory cycles in the device(combustor (C)) at full blown instability. For example, in a device suchas combustor (C) with instability happening at 250 Hz, the samplingwould be at 10 kHz for time duration of 400 ms. The probability of theoperating condition becoming unstable can be defined as

p=N/N _(tot)

The value of p is a measure of the proximity of the operating conditionto instability. In an embodiment, the value of p smoothly increasestowards 1 for an increase of the parameters towards instability.Further, the combustor (C) could be prevented from instability byactivating the control unit 218 when the measured value of p exceeds aset threshold probability as required. In an embodiment, a suitablethreshold is set for the probability of the combustor (C) to attaininstability, such that when the threshold value is obtained, the controlunit 218 suitably ensures that the combustor (C) remains in stableoperating conditions, by controlling various control parameters in thecombustor (C), thereby increasing the stability margin of the combustor(C). FIG. 3 is a graph showing the results of the instability detectorunit based on counting the bursts in the unsteady pressure data obtainedfrom the combustor in a particular configuration.

In an embodiment, the system 200 provided with a unit for earlydetection of onset of instabilities in the combustor (C), by countingthe bursts generated within the combustor (C) includes a signalconditioner 208, threshold logic 210, a comparator 212, a gating signal214, a counter 216, and a control unit 218 as shown in FIG. 5. Thecontrol unit 218 further includes at least one digital to analogconverter 220, an air-flow controller 222 and a fuel flow controller224. FIG. 5 is the system configuration used for early detection ofonset of instabilities in the combustor (C), by counting the burstsgenerated within the combustor (C). The signal Φ(j) generated inside thecombustor (C) is determined by means of appropriate sensors (not shown).Further, the signal conditioner 208 is configured to amplify themeasured signal (Φ(j)). The gating signal 214 generated by an internalgating circuit controls the time duration (t) of signal acquisition. Thethreshold logic 210 includes fixed threshold (ξ), such that when thethreshold logic is applied on the gated signal, the peaks in the signalabove the threshold (ξ) is determined. The comparator 212 is configuredto compare the measured signal Φ(j) with the threshold (ξ) of thesignal. Further, the counter 216 is configured to count the number ofpeaks in the signal Φ(j) above the threshold (ξ). In an embodiment, theoccurrence of burst in the signal increases the amplitude of pressuresignal beyond the threshold value and the threshold logic circuitgenerates a signal indicating the occurrence of peak above thethreshold. Further, the counter 216 counts the number of peaks withinthe gating period (N) and transmits the information based on this number(N) to the control unit 218. The control unit 218 includes the air-flowcontroller 222 that is configured to regulate the functioning of the airflow control valve and the fuel flow controller 224 that is configuredto regulate the functioning of the fuel flow control valve, one or bothof which can be adjusted such that the combustor (C) is prevented frominstabilities. In an embodiment, the signal generated by the controlunit 218 is digital. Further, the digital to analog converter 220 isconfigured to convert the digital signal to analog signal for use in theairflow controller 222 and the fuel flow controller 224.

It should be noted that the aforementioned configuration of system 200is provided for the ease of understanding of the embodiments of theinvention. However, certain embodiments may have a differentconfiguration of the components of the system 200 and certain otherembodiments may exclude certain components of the system 200. Therefore,such embodiments and any modification by addition or exclusion ofcertain components of system 200 and without otherwise deterring theintended function of the system 200 as is apparent from this descriptionand drawings are also within the scope of this invention.

In yet another embodiment, the onset of impending instabilities isdetermined by means of computing the Hurst exponent. For determining theHurst exponent, the signal Φ(j) of length L is divided into a number (n)of non-overlapping segments (x_(i)(j), i=1, 2, . . . , n) of equal span(w). Further, the mean of the signal is subtracted from these segmentsto obtain a cumulative deviate series as,

$m = {\frac{1}{L}{\sum\limits_{j = 1}^{L}\; {\Phi (j)}}}$$y_{i} = {\sum\limits_{j = 1}^{w}\; ( {{x_{i}(j)} - m} )}$

Furthermore, in order to account for local trends in the segments, alocal polynomial fit ( y _(i)) is made to the deviate series (y_(i)).The structure function (S_(w) ^(q)) of order q and span w, is thenobtained as:

$S_{w}^{q} = ( {\frac{1}{w}{\sum\limits_{j = 1}^{w}\; ( {{y_{i}(j)} - \overset{\_}{y}} )^{q}}} )^{\frac{1}{q}}$

The Hurst exponent H² is then obtained as the slope of the linear regimein a log-log plot of S_(w) ² for various span sizes. FIG. 4 is a graphdepicting the variation in Hurst exponent of the unsteady pressure dataobtained from the combustor (C) in a particular configuration for achange in control parameters. The Hurst exponent falls smoothly as thecombustor (C) approaches instability. Instead of using the standardHurst exponent H², the generalized Hurst exponent H^(q) which would givesimilar trends as the standard Hurst exponent could also be used withthe system.

In an embodiment, the system 300 for early detection of onset ofinstabilities in the combustor (C), by means of Hurst exponent includesa signal conditioner 326, an analog to digital converter 328, a digitalto analog converter 330, and a control unit 332. FIG. 6 depicts a system300 for early detection of onset of instabilities in the combustor (C),by means of the Hurst exponent. The signal conditioner 326 is configuredto manipulate the signal (Φ(j)) generated by the measuring device 302,such that it meets the requirements of analog to digital convertor 328.In an embodiment, the signal conditioner 326 is configured to amplifythe signal (Φ(j)) generated by measuring device 302. Further, if thesignal (Φ(j)) generated by the measuring device 302 is analog, theanalog to digital convertor 328 coverts the analog signal to digitalsignal such that the signals (Φ(j)) could be processed in theinstability detection unit 304. The instability detection unit 304operates as per the Hurst exponent algorithm. Further, the control unit332 obtains the information based on the stability of the combustor (C)from the instability detection unit 304 and controls the controlparameter of the combustor (C) such that the instability could beavoided. Further, the digital to analog convertor 330 coverts thedigital signal obtained as the output from the instability detectionunit 304 into the analog signal such that it could be processed by thecontrol unit 332. Further, the control unit 332 obtains the informationbased on the stability of the combustor (C) from the instabilitydetection unit 304 and controls the control parameter of the combustor(C) such that the instability could be avoided. In an embodiment, thecontrol unit 332 is configured to control the parameters of flow controlvalve such that the instability could be avoided.

It should be noted that the aforementioned configuration of system 300is provided for the ease of understanding of the embodiments of theinvention. However, certain embodiments may have a differentconfiguration of the components of the system 300 and certain otherembodiments may exclude certain components of the system 300. Therefore,such embodiments and any modification by addition or exclusion ofcertain components of system 300 and without otherwise deterring theintended function of the system 100 as is apparent from this descriptionand drawings are also within the scope of this invention.

A method for early detection of onset of oscillatory instabilities inpractical devices and controlling various parameters of the device inorder to prevent the device from oscillatory instabilities is explainedherein below. FIG. 7 is a flow chart depicting a method for earlydetection of the onset of oscillatory instabilities in practical devicesand controlling various parameters of the device in order to prevent thedevice from oscillatory instabilities using the system. The method 400includes providing a measuring device in communication with thepractical device (step 402); such that signals corresponding to thedynamics of the practical device are generated by the measuring device(step 404). Further, the stability of the practical device is identifiedby the instability detection unit (step 406). In an embodiment, theinstability detection unit diagnoses the onset of instabilities, by 0-1test method. In another embodiment, the instability detection unitdiagnoses the onset of instabilities, by counting the number of burstsin the measured signal. In yet another embodiment, the instabilitydetection unit diagnoses the onset of instabilities, by means of theHurst exponent. Further, various parameters of the practical device iscontrolled in accordance with the output from the instability detectionunit to maintain the combustor under stable operating conditions (step408).

The foregoing description of the specific embodiments will so fullyreveal the general nature of the embodiments herein that others can, byapplying the current knowledge, readily modify and/or adapt for variousapplications such specific embodiments without departing from thegeneric concept, and, therefore, such adaptations and modificationsshould and are intended to be comprehended within the meaning and rangeof equivalents of the disclosed embodiments. For example, although,certain embodiments herein are related to the system and method thatutilizes the 0-1 test, burst counter and Hurst exponent methods forearly detection of onset of instabilities as they are fast and robust,it should be noted that the system could utilize any other methods thatcould determine the transition to instability through intermittentbursts. For instance, from the variations in the generalized Hurstexponent data (H^(q)), the Holder spectrum could be constructed.Further, a multifractal spectrum width (W) could be calculated by meansof the constructed Holder spectrum. The multifractal spectrum width (W)also has a decreasing trend as the device approaches instability andthereby can be used as an indicator to identify the onset ofinstability. Another possible indicator to identify the onset ofinstability could be obtained from what are known as recurrence plots.By a recurrence quantification analysis, quantities such as laminarity,determinism, trapping time and so on could be obtained. These quantitiesshow trends indicative of the transition. Changes in the values of thelargest Lyapunov exponent are another useful indicator. Furthermore,although certain embodiments of the invention discloses the system andmethod for determining impending instabilities in combustor, it shouldbe noted that the system and method as disclosed in the presentinvention could be used for any other device that is subject tooscillatory instabilities. For example, the generality of the method maybe seen in FIG. 8 wherein the Hurst exponent test is applied to dataacquired from an aeroacoustic system. The test is able to forewarn theonset of oscillatory instabilities well before the actual transition tooscillatory instabilities. Further, it is to be understood that thephraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Therefore, while the embodimentsherein have been described in terms of preferred embodiments, thoseskilled in the art will recognize that the embodiments herein can bepracticed with modification within the spirit and scope of theembodiments as described herein.

Referral numerals 100, 200, 300 System 102, 202, 302 Measuring Device104, 204, 304 Instability detection unit 106, 218, 332 Control unit 108,208, 326 Signal conditioner 210 Threshold logic 128, 328 Analog todigital converter 212 comparator 120, 220, 330 Digital to analogconverter 214 Gating signal 216 Counter 222 Air-flow controller 224 Fuelflow controller 326 Signal conditioner

We claim:
 1. A system to determine impending oscillatory instabilitiesin a device, said system comprising: a measuring unit configured togenerate at least one signal corresponding to dynamics in said device;and an instability detection unit provided in communication with saidmeasuring unit; wherein, said instability detection unit is configuredto diagnose the onset of said impending oscillatory instabilities insaid device based on at least one of intermittency in the signalgenerated by the measuring unit before the onset of instability, orsmooth variations in parameters as the device approaches said impendingoscillatory instabilities, wherein said intermittency is detectedpreceding to a transition from a noisy or chaotic behavior to saidoscillatory instabilities.
 2. The system as claimed in claim 1 furtherincludes a control unit provided in communication with at least one ofsaid measuring unit and said instability detection unit, wherein saidinstability detection unit is configured to generate control signalscorresponding to onset of said impending oscillatory instabilities; andsaid control unit is configured to control said oscillatoryinstabilities that proceed through said intermittency instabilitiesbased on said control signal.
 3. The system as claimed in 1, whereinsaid system is used to detect said impending oscillatory instabilitiesthat proceed through said intermittency in at least one of a combustor,an industrial furnace, a burner, aero-acoustic systems, aero-elasticsystems, aeromechanical systems, air-compression systems and any otherdevice subjected to oscillatory instabilities.
 4. The system as claimedin 1, wherein said system determines the proximity of the system tooscillatory instability that proceeds through said intermittency byperforming at least one of 0-1 test, Burst count test and Hurst exponenttest.
 5. The system as claimed in 1, wherein said system determines anonset of said impending oscillatory instability by using a measure thatcan track the presence of intermittency in said signal.
 6. The system asclaimed in 1, wherein said measuring unit includes a plurality ofsensors that are configured to generate said signal corresponding to thedynamics of the device.
 7. The system as claimed in 6, wherein saidsensor is selected from at least one of an acoustic sensor, a photodiodeand a photomultiplier.
 8. The system as claimed in 1, wherein an analogto digital convertor is integrated with the system to convert at leastone analog signal generated by the measuring unit to at least onedigital signal that could be processed by the instability detectionunit.
 9. The system as claimed in claim 3, wherein a digital to analogconvertor is integrated with the system to convert at least one digitalsignal obtained as an output from the instability detection unit to atleast one analog signal that could be processed by the control unit. 10.The system as claimed in claim 4, wherein the system configured todetect and control said impending oscillatory instabilities in thedevice by performing 0-1 test comprises: said measuring unit provided incommunication with the device and configured to generate at least onesignal (measured signal) corresponding to the dynamics the device; asignal conditioner in communication with said measuring unit andconfigured to amplify the measured signal generated in said measuringunit; an analog to digital convertor connected to said signalconditioner; said instability detection unit provided in communicationwith said analog to digital convertor and configured to generate a valuebetween 0 and 1 based on the proximity of the device to an oscillatoryinstability that proceed through said intermittency in said device; atleast one digital to analog convertor provided in communication with theinstability detection unit; and said control unit provided incommunication with the digital to analog convertor, wherein saidinstability detection unit is configured to generate a value close to 1for noisy/chaotic signals and close to 0 for oscillatory dynamics. 11.The system as claimed in claim 4, wherein the system configured todetect and control said impending oscillatory instability in the devicethat proceed through said intermittency by performing Burst count testcomprises: said measuring unit provided in communication with the deviceand configured to generate at least one signal (measured signal)corresponding to the dynamics in the device; a threshold logic having afixed threshold value for the signal; an internal gating circuitprovided in communication with the threshold logic and configured togenerate a gating signal; a comparator provided in communication withsaid threshold logic and configured to compare measured signal with thethreshold of the signal; a counter provided in communication with saidthreshold logic and configured to count the number of peaks in themeasured signal above the threshold of the signal; and said control unitprovided in communication with said counter and configured to regulatethe functioning of said device, wherein said gating signal controls thetime duration of signal acquisition from the device.
 12. The system asclaimed in 11, wherein said system further includes a signal conditionerprovided in communication with the measuring unit and configured toamplify the measured signal.
 13. The system as claimed in 11, whereinsaid controller is configured to regulate the functioning of at leastone of operating parameters of the device, such that the device isprevented from said oscillatory instabilities that happen through saidintermittency.
 14. The system as claimed in claim 4, wherein the systemconfigured to detect and control said impending oscillatoryinstabilities that proceeds through said intermittency in the device byperforming Hurst exponent test comprises: said measuring unit providedin communication with the device and configured to generate at least onesignal (measured signal) corresponding to the dynamics in the device; asignal conditioner in communication with the measuring unit; an analogto digital convertor connected to said signal conditioner; saidinstability detection unit attached to said analog to digital convertor;at least one digital to analog convertor connected to the instabilitydetection unit; and said control unit attached with the digital toanalog convertor.
 15. A method to determine impending oscillatoryinstabilities in a device, said method comprising: generating at leastone signal (measured signal) corresponding to the dynamics in thedevice; and diagnosing said impending oscillatory instabilities of thedevice by utilizing at least one of intermittency in the signalgenerated by the measuring unit before the onset of instability or saidsmooth variations in parameters as the device approaches said impendingoscillatory instabilities, wherein said intermittency is detectedpreceding a transition from a noisy or chaotic behavior to saidoscillatory instabilities.
 16. The method as claimed in claim 15,further includes generating control signal corresponding to the onset ofsaid impending oscillatory instabilities that proceed through saidintermittency of the device; and controlling said oscillatoryinstabilities that proceed through said intermittency based on saidcontrol signal.
 17. The method as claimed in claim 15, wherein saidimpending oscillatory instabilities that proceed through saidintermittency in the device is diagnosed by performing at least one of0-1 test, Burst count test and Hurst exponent test.
 18. The method asclaimed in 15, wherein said impending oscillatory instabilities of thedevice that proceed through said intermittency is diagnosed by using ameasure that can track the presence of intermittency in said signal. 19.The method as claimed in 15, wherein said process of diagnosing saidimpending oscillatory instabilities by using intermittent burst producedduring said instability further includes: providing a threshold logichaving a fixed threshold value for the signal; providing a gating signalthat is configured to control the time duration of signal acquisitionfrom the device; comparing said measured signal with the threshold ofthe signal; counting the number of peaks in the measured signal abovesaid threshold of the signal; and controlling said impending oscillatoryinstabilities that proceed through said intermittency in the devicebased on the number of peaks.
 20. The system as claimed in 1, whereinsaid system determines said adding impending oscillatory instabilitythat proceeds through said intermittency using at least one ofvariations in measures of fractality, variations in measures ofmultifractality, or variations in measures computed through recurrencequantification, for changes in the operating conditions of said device.21. The method as claimed in 15, wherein said impending oscillatoryinstabilities that proceed through said intermittency of the device isdiagnosed by using at least one of variations in measures of fractality,variations in measures of multifractality or variations in measurescomputed through recurrence quantification, for changes in the operatingconditions of said device.
 22. The method as claimed in claim 15,wherein said process of diagnosing said impending oscillatoryinstabilities in the device by performing 0-1 test comprises: generatingat least one signal (measured signal) corresponding to the dynamics thedevice; amplifying the measured signal generated in said measuring unit;generating a value between 0 and 1 based on the proximity of the deviceto an oscillatory instability that proceed through said intermittency insaid device; and generating a value close to 1 for noisy/chaotic signalsand close to 0 for oscillatory dynamics.
 23. The method as claimed inclaim 15, wherein said process of diagnosing said impending oscillatoryinstabilities that proceeds through said intermittency in the device byperforming Hurst exponent test.